Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965987AbdGTW4O (ORCPT ); Thu, 20 Jul 2017 18:56:14 -0400 Received: from mail-wr0-f194.google.com ([209.85.128.194]:32954 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964863AbdGTW4M (ORCPT ); Thu, 20 Jul 2017 18:56:12 -0400 Subject: Re: cpuidle and cpufreq coupling? To: Peter Zijlstra , "Rafael J. Wysocki" Cc: Viresh Kumar , Linux Kernel Mailing List , Linux PM , "Rafael J. Wysocki" , Markus Mayer , Daniel Lezcano References: <20170720071846.GK352@vireshk-i7> <20170720144530.da6bqpdvt2cvbjjs@hirez.programming.kicks-ass.net> From: Florian Fainelli Message-ID: Date: Thu, 20 Jul 2017 15:56:05 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170720144530.da6bqpdvt2cvbjjs@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1769 Lines: 35 On 07/20/2017 07:45 AM, Peter Zijlstra wrote: > On Thu, Jul 20, 2017 at 11:52:41AM +0200, Rafael J. Wysocki wrote: >> On Thu, Jul 20, 2017 at 9:18 AM, Viresh Kumar wrote: >>> On 20-07-17, 01:17, Rafael J. Wysocki wrote: >>>> On Thu, Jul 20, 2017 at 12:54 AM, Florian Fainelli wrote: >>>>> Hi, >>>>> >>>>> We have a particular ARM CPU design that is drawing quite a lot of >>>>> current upon exit from WFI, and it does so in a way even before the >>>>> first instruction out of WFI is executed. That means we cannot influence >>>>> directly the exit from WFI other than by changing the state in which it >>>>> would be previously entered because of this "dead" time during which the >>>>> internal logic needs to ramp up back where it left. >>>>> >>>>> A naive approach to solving this problem because we have CPU frequency >>>>> scaling available would be to do the following: >>>>> >>>>> - just before entering WFI, switch to a low frequency OPP >>>>> - enter WFI >>>>> - upon exit from WFI, ramp up the frequency back to e.g: highest OPP >>>>> >>>>> Some of the parts that I am not exactly clear on would be: >>>>> >>>>> - would that qualify as a cpuidle governor of some kind that ties in >>>>> which cpufreq? >>>>> - would using cpufreq_driver_fast_switch() be an appropriate API to use >>>>> from outside > > Can your ARM part change OPP without scheduling? Because (for obvious > reasons) the idle thread is not supposed to block. I think it should be able to do that, but I am not sure that if I went through the cpufreq API it would be that straight forward so I may have to re-implement some of the frequency scaling logic outside of cpufreq (or rather make the low-level parts some kind of library I guess).